If the Big 12 felt like they could add 4 teams instead of the 2 they did, I think they would have.
Right now I think they're stuck. There are not many options left for expansion if they decide to expand.
This time around they may have to look at letting go of some of their expectations in order to look at schools in a good market with a large alumni base, etc., etc. That's why I personally feel that UCF is at or near the top of their list, should they choose to expand, AND barring the possibility that a P5 team would somehow, for whatever reason, decide to cut ties with their current conference and go with the Big 12. I'm not really sure who that would be. Iowa? No way. Florida State, Clemson? Yeah right. Arkansas? Now this is getting laughable... you get the point.